1. Field of the Invention
The present invention relates to an amplifier using a field effect transistor (to be referred to as an FET hereinafter) and, more particularly, to an FET amplifier which is used as a low-noise amplifier for satellite communication or the like and realizes noise reduction in a wide band with a compact circuit.
2. Description of the Prior Art
In a low-noise amplifier for satellite communication, a radio wave reaching from a satellite to the ground becomes very weak because of attenuation in the radio wave propagation path between the satellite and the ground or a limitation in transmission capability of the satellite itself. For this reason, noise generated in the amplifier itself is required to be minimized. In recent satellite communication, the frequency band used is broadening along with an increase in traffic, and the amplifier must also cope with a wide band. Therefore, the FET amplifier must have a circuit arrangement capable of simultaneously meeting these two requirements. Studies have been made to meet these requirements by improving the input-side circuit of the FET amplifier.
Generally, when a lossless matching circuit M1 is connected to the input side of an FET 1, as shown in FIG. 1, the noise figure of the FET 1 is determined by the source admittance (Ys=Gs+jBs) of the input circuit, as represented by the equation below:
______________________________________ F = Fo + Rn/Gs{(Gs - Go).sup.2 + (Bs - Bo).sup.2 } F: noise figure defined by the input circuit Fo: optimum noise figure Rn: equivalent input noise resistance Go: conductance giving optimum noise figure Bo: susceptance giving optimum noise figure Gs: conductance of the input circuit (source conduc- tance) Bs: the susceptance of the input circuit (source susceptance) ______________________________________
To constitute a noise optimum FET amplifier on the basis of this equation, it is preferable to make the source admittance (Ys=Gs+jBs) match the noise optimum admittance (Yo=Go+jBo) defined by the FET. Reference symbol M2 in FIG. 1 denotes an output matching circuit.
In the conventional FET amplifier, as shown in FIG. 2, an output terminal 23 of an impedance improving isolator 2 having an input terminal 21 with transformers 24 to 26, a dummy terminal 22, and an output terminal 23 terminated at 50 .OMEGA. is connected to the input terminal of the FET 1 through a .lambda./4 microstrip line 11 and an FET lead inductance 12, thereby attaining matching for realizing optimum noise. Reference numeral 13 denotes an output matching circuit. In this circuit arrangement, however, a loss generated in the matching circuit between the transformer 26 of the isolator output portion and the .lambda./4 microstrip line 11 degrades the noise figure of the FET amplifier. The source impedance of this amplifier exhibits a locus C in the Smith chart of FIG. 3 when noise matching is achieved. However, the source impedance for obtaining optimum noise is normally represented by a locus D in FIG. 3. The two loci have opposite frequency directions and therefore cross each other. An increase in noise at the band ends cannot be avoided, and broad-band noise matching cannot be obtained.
To solve this crossing of the frequency loci after noise matching, Japanese Unexamined Patent Publication No. 63-62405 discloses a microwave amplifier having a circuit arrangement for realizing noise reduction in a wide band. This microwave amplifier has an FET 1 arranged on the surface of a dielectric substrate with a ground conductor formed on its surface and includes an input matching circuit 30 arranged between an input portion IN and the gate of the FET 1, as shown in FIG. 4A. The input matching circuit 30 comprises an open-circuited stub 31 arranged at the input portion, microstrip lines 32 and 33 for connecting the input portion to the gate of the FET, and a short-circuited stub 34 arranged at a point on the microstrip line. Even when a capacitor C is inserted between the ground and the distal end of another microstrip line 35 instead of arranging the short-circuited stub 34, a short end circuit can be realized, as shown in FIG. 4B.
The change in input impedance locus of the amplifier disclosed in this prior art will be described. The reflection coefficient from the gate of the FET to the input side of the input matching circuit 30 is represented by .GAMMA..sub.s, and the reflection coefficients from the respective elements of the input matching circuit 30 to the input side are represented by .GAMMA..sub.1, .GAMMA..sub.2, and .GAMMA..sub.3. The electrical length of the open-circuited stub 31, the microstrip lines 32 and 33, and the short-circuited stub 34 is optimized so that the reflection coefficients .GAMMA..sub.1 to .GAMMA..sub.3 and .GAMMA..sub.s change along loci 41 to 44 on the Smith chart of FIG. 5, respectively. The reflection coefficient .GAMMA..sub.2 which has changed along the locus 42 in the open-circuited stub 31 and the microstrip line 32 changes to the reflection coefficient .GAMMA..sub.3 along the locus 43 in the short-circuited stub 34. The locus .GAMMA..sub.3 is converted into .GAMMA..sub.s (44) by phase rotation along the microstrip line 33 and overlaps an optimum input load reflection coefficient .GAMMA..sub.opt (45) of the FET. Normally, phase rotation of a microstrip line becomes larger as the frequency becomes higher. For this reason, when the locus .GAMMA..sub.3 reaches the position of the locus .GAMMA..sub.s, the length of the locus is small. However, since the displacement from .GAMMA..sub.opt (45) to .GAMMA..sub.s (44) can be reduced as compared to the arrangement shown in FIG. 2, the arrangement shown in FIG. 4A or 4B can realize noise matching in a wide band.
In a low-noise amplifier for satellite communication, to apply the amplifier to the reception system without degrading the characteristics of the amplifier even when the input impedance condition changes in a device arrangement including an amplifier input feed unit and an antenna, an isolator is normally inserted to ensure the input impedance. When the circuit shown in FIG. 4A or 4B is applied for satellite communication, a 50-.OMEGA. matched three-terminal isolator is connected to the input side. Therefore, the noise figure drops by the loss of the isolator.
In the improved amplifier shown in FIG. 4A or 4B, broadening of the noise figure in terms of frequency can be attained to some extent, though the noise figure as an absolute value is poor. When a 50-.OMEGA. matched isolator is added to the prior art, the noise figure suffers by the loss of the isolator. The reason for this is as follows. Since a plurality of devices (open-circuited and short-circuited stubs) associated with input broad-band matching and microstrip lines are formed on the dielectric substrate, noise degradation is caused by an increase in insertion loss. The scale of the circuit including the input matching circuit becomes large, hampering a size reduction of the device. This is because the input matching circuit is constituted and realized by microstrip lines as a distributed parameter circuit. As the frequency becomes lower, the electrical length becomes larger, and the scale of the circuit on the substrate also tends to be larger.